1. Field of the Invention
The invention relates to inertialess radar scanning techniques in general, and more specifically to individually controlled radiating elements particularly adapted for use in phase scanning arrays.
2. Description of the Prior Art
Since the earliest times of radar system development, array antennas have been known per se, and have been used for the formation of sharply directive beams. Array antenna characteristics are determined by the geometric position of the radiators (elements) and the amplitude and phase of their individual excitations.
Intermediate radar developments, facilitated by the development of the magnetron and other high powered microwave transmitters, had the effect of pushing the commonly used radar frequencies upward. At those higher frequencies, simpler antennas became practical. Such simpler antennas usually included shaped (parabolic) reflectors illuminated by horn feed or other simple primary antenna.
As the radar art advanced, electronic (inertialess) scanning became important for a number of reasons, including scanning speed and the capability for random or programmed beam pointing. Since the development of electronically-controlled phase shifters and switches, attention has been redirected toward the array type antenna in which each radiating element can be individually electronically controlled. The text "Radar Handbook" by Merrill I. Skolnik, McGraw Hill (1970) provides a relatively current general background in respect to the subject of array antennas in general, particularly in Chapter 11 thereof.
Chapter 12 of the above-referenced textbook is devoted to "Phase Shifters for Arrays", such controllable phase shifting devices being a key element in the phased array prior art. The capability for rapidly and accurately switching beams thus afforded permits a radar to perform multiple functions interlaced in time, or even simultaneously. An electronically steered array radar may track a great multiplicity of targets, illuminate a number of targets with RF energy for the purpose of guiding missles toward them, perform wide-angle search with automatic target selection to enable selected target tracking and may even act as a communication system directing high gain beams toward distant receivers and/or transmitters. Accordingly, the importance of the phase-scanned array as a modern radar tool, is very great indeed.
In a phased-array system, a number of unique problem areas exist which have been at best, only partially solved and then at great expense and complexity, in accordance with prior art technology. These problems are typically concerned with the local feed, the phase shifters, the elements, and the type and quality of polarization.
The manner in which signal is distributed from a common input to the sub-array and thence to the elements of a particular array has a substantial effect on the total cost and performance of the array. Most arrays are designed from the following points of view: (1) An attempt is made to match the element active impedance, which varies with scan angle. (2) The element is driven from a matched phase shifter. (3) The group of elements is driven from a feed with matched, isolated, output ports.
The rational for the "matching" design approach is that a matched system results in maximum power transfer. Even in a well-designed antenna with wide scanning requirements, the element VSWR is likely however, to be not less than 6dB. It is necessary for the output ports of the feed to be well matched, because multiple reflections between the element and the feed result in problems as follows: (1) For reciprocal phase shifters, high spurious side-lobes are generated due to multiple passes of the reflected signals therethrough; these being re-radiated in spurious directions. (2) For non-reciprocal phase shifters, substantial variations in gain is experienced due to multiple passes of the reflected signals through the phase shifters, these being re-radiated in the main-beam direction.
The prior art design philosophy has resulted in systems with only moderate performance. The cost, moreover, has been high as each component part must be tightly controlled. The size and weight of the array is frequently a problem because it requires three basic elements in series for each radiating element.
The manner in which the present invention deals with the problems of the prior art to produce an integral antenna element and phase shifter will be evident as this description proceeds.